Crude Tracking Device From a GPS and Two-Way Radios

So, I wanted to get a tracking device. As soon as I looked into the market, I realized the prices for one of those things begins at an arm, and goes up to a leg or more! The madness must be stopped! Surely the principles of knowing where something is could be used without paying out the nose for it. After some hair pulling, this idea was born. This took a lot of trial and error before I was able to hammer out what it has become for me today, and for all that, it is just workable enough for my purposes. Therefore, if you can think of a way this concept could be made better, please share! I've built on the shoulders of giants, and I expect you to do the same. This idea belongs to the people! Information is free! Insert an inspiring slogan here!
The nature of this tracking device is extremely limited and technical, but if you can hack it, this really works -- and that in itself is really exciting. The limitations are at times frustrating:

• The 2-way radios necessitate a linear or nearly linear sight on your tracking device (which in many cases may spoil the whole point anyway).
• You must be closer to the transmitter than the nominal range of your radios.
• The tracker can get bulky (think about it -- you're basically lashing together two electronic devices that were originally meant to work separately).
• This one is the worst limitation: you will need to be able to manually sift through the static garbage that radios usually emit to find the data your GPS is trying to send. If you are a total 1337 h4ck3r, you may be able to write a program that pulls it out automatically and feeds it into Google Earth. (If, incidentally, you do accomplish this, tell me what to do to gain your favor when you take over the world.) Beware! Radio static can get really random, and will even imitate the data you are after, often giving you misleading results.

So far, for all my test runs, this device has about a 60 percent accuracy. Yikes.

To quote, with a slight change, someone who inspired me to think this way: "If you can't modify it, you don't own it!"
Building on that, the lesson I have learned and hope to teach others is that something's label should not limit the way we use it.

Get a GPS that you can plug into a computer. I used a Garmin GPS 72. I am of the opinion that Garmins are the best and most reliable, as well as the easiest to hack. If you insist on using a different kind, be prepared to figure out the I/O stuff yourself. At any rate, I suppose this project is possible with any sort of GPS unit, provided it can send info at a low baud rate.
Get a cable that connects the GPS to a computer. For my project, I used a COM port plug, since I already had it lying around, but I suspect using a USB connector would have been slightly easier.
Get a pair of walkie-talkies (Americanese for 2-way radios) that have an impressive range. For this example, I am using a pair that boasts a 12-mile range.
Other materials included in this project: Wire, solder, batteries, a computer, and glue or epoxy. Optional: something to track.
Tools you will use include: wire cutters, wire strippers, screwdrivers, pliers, a soldering iron, a DMM (Digital Multi-Meter), scissors, an undefiled radio, and the user manual of your GPS.
Find a quiet, peaceful place to fine-tune this. Several of the steps are bound to be frustrating for a first-timer, as they were to me.

Start with the simple stuff: Cut that cable in half. Strip it nice and clean to expose all the wires in their full glory. if your cable is like mine, there is room for four wires, but you'll only find three. In fact, you'll technically only need two of them for the transmitter half!
I use scissors to cut and strip my wires, but that is because 1) I am too lazy to get the wire cutters and strippers out, and 2) I don't care about how dull I make the scissors. I find them to be both convenient and dangerous. Be careful!
Next, open up the radios. Spend some time with their insides. get to know them. Maybe take them out for a nice dinner or a walk on the beach. Carefully solder out the speaker on one, and the microphone on the other. I tore them off using pliers, which in retrospect was a stupid risk, and I was lucky nothing bad happened. Make a note of the contacts on the circuit board where those components had been. You'll be using those contacts later. Toss the speaker and the microphone into your spare parts bin.

On the GPS plug end of the cable we have hacked, we are going to solder two of its wires onto each of the two microphone leads (think of the GPS talking into the radio). But first, we need to figure out exactly which wires go where! You'll need to refer to your user manual to find which pin on the com array in the GPS is used for what, and then use your DMM to determine which wire goes to which pin. We are interested in the ground pin and the data out pin. On my GPS 72, those are the pins on the right and the bottom of the array.
If this info isn't in your manual, you can determine which two wires you are interested in because a strong current goes across them when your GPS is attempting to transmit information. When applied to the tongue, it feels like quite a kick, while combinations of any other two wires are barely noticeable. Don't shock your tongue for too long, or it will go numb and probably even get injured.
Also keep in mind that the polarity of the contacts is important, so don't make the connection permanent. Once you are sure the contacts are in the best position and polarity possible, solder them good and then glue them down for sturdiness. I used hot glue, but I think epoxy would have been a lot better.

Now we're going to do the same basic concept with the receiving radio. Find pins 2, 3, and 5 on the COM end of the severed cable, and relate them to the wires at the other end using your DMM. I found that the COM port's pin holes were too small for my DMM probes, so I twisted a skinny bit of copper wire around them to stick in there.
Twist together and solder the wires that correspond to pins 2 and 3, so that you get one node out of the two pins. I'm not sure why the receiver won't work if you don't do this, but it took me six hours of fiddling with it before I figured that out.
Connect the wires to the contacts in the receiver that used to host the speaker. Again, polarity in this is important, so test everything before making it permanent.

Once you have everything set up for testing, download and install Earth Bridge. What a wonderful program! Although it is meant to be used as a liaison with Google Earth, you're going to have to act as its middle man, since it is not used to hearing the static of radio signals. Download and install Google Earth if you don't already have it (as if!). Set your GPS and Earth Bridge at the lowest baud rate possible. In my case, this was 1200 baud, which is barely acceptable for this project. Having a low baud rate gives Earth Bridge a better chance of making sure a part of what it is hearing is an actual data bit instead of static. Tell Earth Bridge in the Preferences tab to correspond to the same baud rate, and unclick all the options. What you're after here is the raw text in the GPS Status tab.

Connect the transmitter to your GPS. Connect the receiver to your computer. Turn everything on. Tune your radios to the same channel (Preferably one that nobody normally uses).
Initially, my GPS com signal was strong enough to tell the transmitting radio when to transmit and when to relax. I'm not sure why it was fine before, but now the transmitter omits the North coordinates unless the PTT button is held down, so I'll have to tape it down when tracking something.
Once all this is set up and the radios are both on the same channel, make sure the receiver is on full volume so Earth Bridge can hear it. Click the "Connect" button and it should immediately start spewing garbage onto the text box. If you study this garbage for a while, you should notice a pattern that follows specific rules about what the GPS is trying to say. You can use these rules to sift through the garbled mess to pick out coordinates. (If you happen to have a Garmin sending text at 1200 baud, and for some reason also have WordPerfect 12 on your computer, I have written a few rule-based macros you can use to sift through the radio junk quickly!)
When you start to recognize what your coordinates look like, you can type them in as a placemark in Google Earth and see precisely where your tracking device is! After a lot of practice, this process can be compressed into a 2-minute frenzy that makes you look like a computer hacker from the 80s (wear steampunk goggles and gloves for full effect)!
Backing up a bit, if you have everything set up but are not receiving any signal, or just a completely unusable signal, it is very likely that the polarity on your receiver or transmitter is reversed. Try reversing the contacts on either device (or, in my case, both!) until you receive a usable signal.
Another good way to test the transmitter is by using a radio you have not hacked apart. Listening to the transmitter through the test radio should sound a bit like a broadcast from the Emergency Broadcast System (which makes me wonder if that is a string of data they're broadcasting). After tweaking with everything for a while, you'll get a really good idea of what it is supposed to sound like.

When sending off a tracking device, it is important to think ahead how long you plan to track, as well as how far you are willing to go to keep tabs on it. This will help you decide what sort of batteries to use, as well as how portable you'll want to be. I had my receiver hooked up to my laptop, which in turn was hooked up to an inverter in my car.
I tested my setup with rechargeable batteries. My radios take 3 AAA batteries, and my GPS takes 2 AA batteries. I used AAA batteries rated at 1000 mAh, and AA batteries rated at 2600 mAh. Initial calculations led me to predict that the transmitting radio would last 2.5 hours. Based on this, I'd recommend that your transmitting radio batteries have the highest mAh rating possible. If you're doing long-term tracking, this isn't for you.
Interestingly, the receiving radio hardly uses its batteries at all. I gave mine a fresh set of alkaline AAAs at the beginning of this whole ordeal and haven't changed them once.
I gave my transmitter a second test a bit later, using D-sized 12000 mAh batteries (yikes!). They survived more than 24 hours of transmitting, but I'm not sure exactly when they died because I left it alone for too long. This was when the GPS did not omit the North coordinates, so I'll have to do another test with the talk button taped down and post the results here.